This invention generally relates to printing devices and methods, and more particularly relates to an image forming system and method for forming an image on a recording medium, the system including a thermo-mechanically activated drop-on-demand (DOD) pagewidth inkjet printhead which conserves power.
Ink jet printing is recognized as a prominent contender in digitally controlled, electronic printing because of its non-impact, low-noise characteristics, use of plain paper and avoidance of toner transfers and fixing. For these reasons, drop-on-demand printers have achieved commercial success for home and office use.
A drop-on-demand inkjet printer is disclosed in U.S. Pat. No. 3,946,398, which issued to Kyser et al. in 1970. This patent discloses a drop-on-demand ink jet printer which applies a high voltage to a piezoelectric crystal, causing the crystal to bend. As the crystal bends, pressure is applied on an ink reservoir for jetting ink drops on demand. Other types of piezoelectric drop-on-demand printers utilize piezoelectric crystals in push mode, shear mode, and squeeze mode. However, the patterning of piezoelectric crystal and the complex high voltage drive circuitry necessary to drive each printer nozzle are disadvantageous to cost effective manufacturability and performance. Also, the relatively large size of the piezo crystal prevents close nozzle spacing thereby making it difficult for this technology to be used to design high resolution page width printheads.
Great Britain Pat. No. 2,007,162, which issued to Endo et al. in 1979, discloses an electrothermal drop-on-demand ink jet printer that applies a power pulse to an electrothermal heater which is in thermal contact with water based ink in a nozzle. A small quantity of the ink rapidly evaporates, forming a bubble which causes drops of ink to be ejected from small apertures along an edge of a heater substrate. This technology is known as thermal ink jet printing.
More specifically, thermal ink jet printing typically requires heater energy of approximately 20 .mu.J over a period of approximately 2 .mu.sec to heat the ink to a temperature of 280-400.degree. C. which causes rapid, homogeneous formation of a bubble. The rapid bubble formation provides momentum for drop ejection. Collapse of the bubble causes a pressure pulse on the thin film heater materials due to the implosion of the bubble. However, the high temperatures needed with this device necessitates use of special inks, complicates driver electronics, and precipitates deterioration of heater elements through kogation, which is the accumulation of ink combustion by-products that encrust the heater with debris. Such encrusted debris interferes with thermal efficiency of the heater. In addition, such encrusted debris may migrate to the ink meniscus to undesirably alter the viscous and chemical properties of the ink meniscus. Also, 10 Watt active power consumption of each heater prevents manufacture of low cost, high speed pagewidth printheads.
Another inkjet printing device is disclosed in commonly assigned U.S. patent application Ser. No. 08/621,754 filed on Mar. 22, 1996, in the name of Kia Silverbrook. The Silverbrook device provides a liquid printing system incorporating nozzles having a meniscus poised at positive pressure so that the meniscus extends from a nozzle tip. A heater surrounding the nozzle tip applies heat to the edge of the meniscus. This technique provides a drop-on-demand printing system wherein means (i.e., the heater) of selecting drops to be ejected produces a difference in meniscus position between selected drops and drops which are not selected, but which is insufficient to cause the ink drops to overcome the ink surface tension and separate from the body of ink. In this regard, an additional means is provided to cause separation of the selected drops from the body of ink. Such means of separation uses surface tension reduction and requires specialized inks. In addition, poising the meniscus at a positive pressure may cause nozzle leakage due to contamination present on any single nozzle. In this regard, application of an electric field or adjustment of receiver proximity is used to cause separation of the selected drops from the body of the ink. However, the electric field strength needed to separate the selected drop is above the value for breakdown in air so that close spacing between nozzle and receiver is needed; but, there is still the possibility of arcing. Causing separation of the drop using proximity mode, for which the paper receiver must be in close proximity to the orifice in order to separate the drop from the orifice, is unreliable due to the presence of relatively large dust particles typically found in an uncontrolled environment.
Yet another inkjet printing system is disclosed in commonly assigned U.S. patent application Ser. No. 09/017,827 (Attorney Docket No. 77,182) filed Feb. 3, 1998, in the name of Lebens et al. The Lebens device provides an image forming apparatus incorporating an ink jet printhead where a single transducer is used to periodically oscillate a body of ink in order to poise an ink drop and form a meniscus. The Lebens device further comprises an ink drop separator associated with the transducer for lowering the surface tension of the meniscus to separate the ink drop from the ink body. The device of the Lebens et al. patent can lead to edge effects in a large printheads, such as a pagewidth ink jet printhead, due to non-uniform poising of drops. In this case, use of a single oscillator can lead to menisci forming in the middle of the printhead and none forming at the ends of the printhead.
Consequently, there remains a widely recognized need for an ink jet printing technique, providing such advantages as reduced cost, pagewidth printing capability, increased speed, higher quality, greater reliability, reduced printhead edge effects, less power usage, and simplicity of construction and operation. The invention, which includes a thermo-mechanically activated DOD (Drop On Demand) printhead, obtains such advantages.
Therefore, there has been a long-felt need to provide a pagewidth image forming system and method for forming an image on a recording medium, which system is capable of conserving power.